1. Technical Field
The present disclosure relates to a circuit and method of detecting a saturation level of an image sensor and an image sensor including the saturation level detecting circuit and, more particularly, to a circuit and method of detecting a saturation level of an image sensor including a photodiode using a black pixel circuit included in the image sensor.
2. Discussion of the Related Art
Image sensors used in cellular phone cameras and digital cameras include CMOS image sensors and charge-coupled device (CCD) image sensors. The image sensors capture images and output video signals corresponding to the captured images.
FIG. 1 illustrates a conventional CMOS image sensor. The CMOS image sensor includes a module lens 110 for condensing light and a semiconductor chip 120 generating a video signal corresponding to the condensed light. The chip 120 includes an image pixel region 130 including an image pixel circuit, an optical black region 140 including a black pixel circuit for removing an error caused by a voltage offset or by heat, a row driver 160 for driving pixels arranged in rows, and an analog-digital converter 150 for converting analog video signals received from pixels arranged in columns into digital video signals.
FIG. 2 illustrates the image pixel circuit 131 and the black pixel circuit 141 respectively included in the image pixel region 130 and the optical black region 140 of FIG. 1. Referring to FIG. 2, the image pixel circuit 131 and the black pixel circuit 141 essentially have the same circuit components. More specifically, each of the image pixel circuit 131 and the black pixel circuit 141 includes a photodiode PD receiving input light Lin, a transfer switch TTr receiving a transfer control signal TC, a floating diffusion node FDN, a voltage follower FTr, a select switch STr receiving a select control signal SC, and a reset switch RTr receiving a reset control signal RC. One terminal of the photodiode PD is connected to the source of a reference voltage, which is ground voltage GND in FIG. 2, and one terminal of the reset switch RTr is connected to a source of a power voltage VDD. A voltage signal Vout output from the pixel circuit 131 or 141 is input to an analog-digital converter ADC (150 of FIG. 1).
While the image pixel circuit 131 and the black pixel circuit 141 have the same configuration, the photodiode PD of the image pixel circuit 131 is exposed to light, whereas the photodiode PD of the black pixel circuit 141 is blocked from light by a light-shielding layer.
The photodiode PD of the image pixel circuit 131 generates charges caused by heat as well as charges caused by photoelectric conversion. Thus, to obtain a correct amount of charges caused by photoelectric conversion, the amount of the charges caused by heat should be subtracted from the total amount of generated charges. The photodiode PD of the black pixel circuit 141 generates only charges caused by heat, because charges caused by photoelectric conversion are not generated due to the light-shielding layer. Accordingly, the analog-digital converter ADC (150 of FIG. 1) that receives voltage signals from the image pixel circuit 131 and the black pixel circuit 141 can output a digital video signal corresponding to the amount of charges caused by photoelectric conversion by subtracting the voltage signal output from the black pixel circuit 141 from the voltage signal output from the image pixel circuit 131.
The black pixel circuit 141 is included in the optical black region 140 to remove any error caused by heat and to eliminate the effect of an offset existing in the photodiode PD of the image pixel circuit 131 and the photodiode PD of the black pixel circuit 141.
A photodiode has a saturation level that varies for wafers or chips due to manufacturing environments or other variables. Thus, image sensors have different saturation levels. Accordingly, the image sensors do not use their respective output ranges but use an output range that can be commonly applied thereto to operate stably. That is, the dynamic range of an image sensor is narrower than the actual output range in order to secure a stable operation margin of the image sensor.
Reducing the dynamic range of an image sensor, however, requires expensive devices for detecting levels that become finer in the reduced dynamic range. This is a problem different from a problem that the dynamic range become narrower because an operating voltage of the image sensor becomes lower.